The use of coronary vasoactive drugs during coronary arteriography will be investigated in animals, in an effort to differentiate hemodynamically significant and relatively inert coronary artery stenoses. Intra-coronary infusion of adenosine triphosphate previously proved to be an optimal vasodilatory agent for these purposes; intra-coronary injection of angiotensin II was an effective, well tolerated vasoconstrictive agent. Stenoses will be produced in the circumflex coronary artery of dogs by typing a single ligature around both the circumflex artery and a juxtaposed No. 17 or 18 gauge cannula, and then removing the cannula. Prior experience has indicated that such stenoses reduce blood flow by about 25 and 50 percent respectively. Coronary arteriography, both conventional and pharmacologically modified, will be performed before, immediately after, and at weekly or twice weekly intervals. Collateral flow becomes obvious angiographically within one to four weeks after production of such stenoses. It is anticipated that at some point in the development of these collateral vessels, collateral flow only be apparent by noting different flow patterns on conventional and pharmaco-coronary arteriograms. Such redistribution of myocardial blood flow may provide the most sensitive method for recognizing the hemodynamic significance of coronary artery stenoses.